ssc-367 - Ship Structure Committee

8.2.2
VIV Response and Stresses
A strategy based on avoidance of VIV is quite feasible for most
marine structures. Primarystructuralmembers are usually designed
to be sturdy enough that they are not susceptible toVIV. However,
some secondary or non-structuralmembers may be susceptible to VIV
in water and in air. An empirical approach proposed by DnV
(Reference 8.7) does not account for the nonlinear relationship
betweenresponseanddamping,therebyyielding conservativeresponse
amplitudes and stresses. To predict response amplitudes more
reliably an approach based on Hallam et at (Reference 8.9) is
recommended.
Cross-flow oscillations due to wind action may not always be
preventable, requiring the members to have sufficient resistance.
An empirical formulation based on a procedure by Engineering
Sciences Data’ Unit ESDU (Reference 8.6) that accounts for
interaction between vortices shed and forces induced is
recommended. This procedure and the basis for estimating maximum
bending stresses for different boundaryconditions are discussed in
Sections D.4 andD.5 of Appendix D.
8.3
FATIGUE DAMAGE ASSESSMENT
All members susceptible to VIV should be assessed for fatigue
damage. First, the fatigue damage due to VIV is calculated. Then
a global fatigue analyses is performed and fatigue determined for
all critical members. The total fatigue damage is equal to the sum
of local (VIV) and global fatigue damage on each member.
Step-by-step determination of both local and global fatigue damage
is discussed further in Section D.6 of Appendix D. Application of
the procedurecould indicatethat the fatigue life is expended after
relatively small number of oscillations, requiring corrective
measures to be taken either in the design process or during
fabrication (devices,spoilers, etc.).
8-5